Mycobacterium bovis strain BCG and components of BCG were previously shown to have prophylactic and therapeutic effects against the line-10, transplantable guinea pig hepatocarcinoma. The present study was undertaken to investigate the possibility that BCG and some neoplastic cells have common antigenic components. Rabbits were immunized with BCG sonicates or line-10 tumor cells. Other rabbits were immunized with line-1 cells, a tumor with antigenic characteristics different from those of line-10. The binding of antibodies in these sera to soluble radiolabeled antigens prepared from BCG and line-10 cells was studied by precipitation of radiolabeled antigen-antibody complexes with antirabbit immunoglobulin. Antibodies in sera from rabbits immunized with BCG and line-10 cells bound both the labeled BCG and line-10 antigens. Antibodies in sera from rabbits immunized with line-1 cells bound the line-10 but not the BCG antigen. Inhibition studies confirmed the specificity of these reactions. Inhibition experiments also showed that antigenic components are shared by BCGand extracts of Rous sarcoma cells but not by extracts of a mouse plasma-cell tumor. Whether the antigens shared by BCG and line-10 tumor cells are identical with tumor-specific antigens and whether they have a function in the outcome of immunotherapy remain open questions.